1. Field of the Invention
The present invention relates to a backlight unit, and more particularly, to a backlight unit for a liquid crystal display device. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for improving an efficiency of lamps and permitting a better wiring arrangement for the lamps.
2. Discussion of the Related Art
A cathode ray tube (CRT), mostly used for monitors for TV receivers, measuring instruments, information terminals, and so on, cannot satisfy the requirements for reducing size and weight of electronic products due to heavy weight and hefty size.
Thus, the size and the weight of the CRT have certain limitations in the trend of compact size and light weight of electronic products.
The devices that are expected to replace the CRTs include liquid crystal displays (LCD) using an electro-optical effect, plasma display panels (PDP) using gaseous discharge, electro-luminescence (EL) displays using an electro-luminescence effect, and the like. Among the above devices, researches on the LCD are the most active.
For replacing the CRTs, the LCDs have been actively developed for reducing size, weight, and power consumption of displays. Recently, they are fully developed for monitors in laptop computers, monitors in desktop computers, and large-sized information displays.
Most of the LCDs are light receiving devices in which the control of light provided from the outside of the device is required for displaying a picture. Thus, a separate light source for providing light to the LCD panel (i.e., a backlight unit) is required.
In general, the backlight unit used as a light source of the LCD is comprised of cylindrical lamps of an edge light type or a direct type.
In the edge light type, a lamp unit is placed at a side of the light plate, which guides the light. The lamp unit is provided with a lamp for emitting the light, and lamp holders inserted in both sides of the lamp for protection of the lamp. A lamp reflecting plate is inserted in a side surface of the light plate to surround the outside surface of the lamp for reflecting the light from the lamp toward the light plate.
The edge light type having the lamp unit provided to a side surface of the light plate has been employed in relatively small-sized LCDs, such as laptop computers and desktop computers. The edge light type has a good uniformity of light and a long lifetime, and has an advantage in fabricating thin LCDs.
On the other hand, the direct type has been developed mainly for the size of an LCD greater than 20″. The direct type is provided with a plurality of lamps arranged in line under a diffuse plate for directing the light to a front surface of the LCD panel, directly.
Since the direct type has a light utilization efficiency higher than the edge light type, the direct type is employed in large-sized LCDs.
A related art backlight unit will be described with reference to the attached drawings.
FIG. 1 illustrates a perspective view of a direct type backlight unit according to a related art. FIG. 2 illustrates a plane view of a lamp arrangement of the direct type backlight unit of FIG. 1. FIG. 3 illustrates a rear view of a backlight unit having inverters for applying an electrical signal to the lamps in FIG. 2. FIG. 4 illustrates a plane view of another related art lamp arrangement of a backlight unit.
Referring to FIGS. 1 and 2, the related art backlight unit is provided with a plurality of lamps 1, an outside case 3 for holding and supporting the lamps 1, and light scattering means 5a, 5b, and 5c between the lamps 1 and an LCD panel (not shown).
The light scattering means 5a, 5b, and 5c is provided with a plurality of diffusion sheets and diffusion plates between the lamps 1 and the LCD panel for enhancing a light scattering effect to prevent the lamps from appearing on the screen of the LCD and provide a uniform luminance distribution.
There is a reflection plate 7 on an inside surface of the outside case 3 for directing the light from the lamps 1 to the display part of the LCD panel for maximizing a light utilization efficiency.
The lamp 1 is a cold cathode fluorescent lamp (CCFL) provided with electrode parts 2a and 2b at the opposite ends of the inside of a tube, each connected to a power line 9a or 9b. The opposite ends of the lamp 1 are inserted in holes formed in the opposite surfaces of the outside case 3.
When a voltage is provided to the electrode parts 2a and 2b through the power lines 9a and 9b, each lamp 1 emits the light. The voltage provided to the power lines is supplied from an inverter 30 provided in one side portion of the back of the outside case 3, as shown in FIG. 3.
In the power lines 9a and 9b, there are a high voltage power line 31 and a low voltage power line 32 depending on voltages provided thereto. That is, the high voltage power line 31 and the low voltage power line 32 from the opposite ends of the lamp 1 are connected to each terminal of the inverter 30 connector for receiving voltages.
For minimizing electrical influences to the LCD panel and external device, the length of the high voltage power line 31 should be shorter than that of the low voltage power line 32.
In the direct type backlight unit of the related art, the length of the lamp 1 is substantially equal to that of the front light emission surface of the backlight unit. Therefore, the larger the size of the light emission surface, the longer the length of the lamp 1. For example, currently the LCD with the size over 30″ to 40″ requires a length of the lamp longer than 700 mm.
Thus, when the lamp is too long, fabrication of the lamp becomes more difficult. In addition, assembly of the lamp with the backlight unit becomes complicated, and the unit is susceptible to the external impact. Moreover, a luminance of the lamp becomes not uniform, and the problem of picture quality deterioration (EMI: electro magnetic interference) occurs due to instability and the electric influence from the driving circuit of the LCD. This is because a high turn-on voltage and a sustain voltage are required.
Due to such a problem, in some of the direct type backlight units, the lamps 40 are arranged in the vertical direction from the light emission surface. Similarly, the electrode parts 42a and 42b are provided at the opposite ends of an inside of a tube of the lamp 40, and the power lines 43a and 43b are connected to the electrode parts 42a and 42b, respectively. The reference numeral ‘41’ denotes a reflection plate.
However, the arrangement of the lamps 40 in the vertical direction of the light emission surface causes mercury in the tube of the lamp 40 to move to the bottom thereof, thereby deteriorating performance of the lamp 40 when the backlight unit is used for a long time.